Many non-volatile semiconductor memory devices incorporate NAND flash memory cells. The NAND flash memory cell includes a charge-storage transistor including a charge-storage layer formed on an insulation film between a channel layer and a gate electrode. When charges accumulate in the charge-storage layer, the threshold voltage of the charge-storage transistor may be changed, as a result of which, information may be read in and out. For high-capacity, it is necessary to further refine the charge-storage transistor but the insulation film is thinned, which increases the leakage current and weakens the charge holding capacity. As a result, it is more and more difficult to use the charge-storage transistor for high-capacity.
In a CBRAM, at the cross-point of word lines (WLs) and bit lines (BLs), a variable resistance element is used as a memory element. With the CBRAM, which relies on the variance in the ohmic value of the memory cells, compared with the charge-storage memory cells, the retention of memory is seldom a problem. Therefore, high-capacity CBRAM with cross-point memory cells is a possibility. In particular, CBRAM, as cross-point CBRAM, is anticipated. Metallic ions in the diffusion layer between two electrodes of CBRAM memory cells may diffuse out thus changing the ohmic value of the memory cells by controlling the current path connecting the two electrodes. Further, the CBRAM has rectification characteristics as well as variable resistance characteristics. In a cross-point non-volatile semiconductor device, generally, since a reverse bias voltage is applied to a non-selected memory cell upon read out, it is necessary to suppress a current from the non-selected memory cell. Thus, it is necessary to connect a rectifying element such as a diode to the memory cell in series. CBRAM memory cells have the function of rectification, so it is not necessary to connect the rectifier element. Therefore, the structure of CBRAM memory cells is simplified and high-capacity is possible.
However, when the memory cells are smaller than 10 nm in size, the diffusion of metallic ions makes the control of the current path more difficult. When the metallic ions diffuse excessively to the diffusion layer and if the memory cells alternate between ON and OFF states, the deviation of the switch may increase and the endurance of the switching angle may be weakened. If the size of the memory cells decreases, the metallic ions will be diffused excessively. In addition, the rectification characteristics are also deteriorated due to lower resistance in the rectification properties that will allow the reverse current to increase. For this reason, CBRAM with memory cells smaller than 10 nm in size that have good rectification and switching characteristics is worth pursuing.